The present invention relates to ink jet printing and, more particularly, to an ink jet printer in which printer operation and reliability at start-up and shutdown are enhanced.
Ink jet printers accomplish printing by depositing drops of ink on a print receiving medium in a pattern such that a print image is collectively formed by the drops. Typically, an ink jet printer includes a print head which defines a fluid reservoir to which electrically conductive ink is supplied. At least one orifice, defined by an orifice plate or similar structure, communicates with the fluid reservoir. It is common that an orifice plate will define a plurality of orifices which are arranged in one or more rows. Ink is forced under pressure through each orifice and emerges from the orifice as a fluid filiment. Pressure varicosities are generated in the fluid filament or filaments by mechanical stimulation of the orifice plate or by generating pressure waves which travel through the ink in the fluid reservoir. The fluid filaments are therefore caused to breakup into streams of ink drops of substantially uniform size and spacing.
Charge electrodes are positioned beneath the orifice plate, adjacent the tips of the fluid filaments. Electrical charge potentials, selectively applied to the charge electrodes, induce corresponding charges of opposite polarity on the drops as they are formed from the filament tips. The drops then pass downwardly through a deflection field, with the charged drops being deflected by the field and the uncharged drops passing through the field in non-deflected trajectories. The amount of deflection experienced by a drop is dependent upon a number of factors, including the level of charge carried by the drop, the strength of the deflection field, the mass of the drop, and the time required for the drop to traverse the field.
During the start-up process the pressure of the ink in the fluid reservoir is increased over a short but finite length of time. Until the pressure reaches the normal operating pressure for the print head, the fluid flow characteristics of the jet are unpredicatable and, additionally, the stimulation system may not be effective in producing breakup of the drops. As a consequence, the breakup timing, size of the drops formed, and initial trajectories of the drops will vary unpredictably.
There is, therefore, a possibility that large amounts of ink may be deposited upon the charge electrodes and upon the deflection field electrode structure of the printer during start-up. If this occurs, the electrically conductive ink tends to short out the charge electrodes and the deflection electrode structure, and may also interfere with the trajectories of the jets once stable operation is attained. Additionally, ink may be deposited on the print receiving medium transport and spoil subsequently printed copies carried by the transport.
Similar problems are encountered at shutdown of the printer. As the pressure of the ink in the fluid reservoir is reduced and fluid flow through the orifices is terminated, the jets once again become unstable and difficult to control.
Several different approaches have been taken to overcome the problems presented by jet instability at start-up and shutdown. As shown in Van Bremen et al, U.S. Pat. No. 4,081,804, a print head has been mounted over a drip pan at start-up to collect drops formed from the fluid filiments until after the jets become stable. A print receiving medium is then transported between the print head and the drip pan, and printing is initiated.
A notched charge electrode plate is shown in IBM Technical Disclosure Bulletin, Vol. 20, No. 1, June 1977, pages 33 and 34. The charge electrode plate may be pivoted into an operating position only after start-up is completed. During the start-up operation, the charge electrodes are removed from the region of drop formation, thereby reducing wetting of the charge electrodes. In an alternative arrangement, the charge electrode plate may be translated, rather than pivoted, into its operating position after start-up. While reducing fouling of the charge electrodes, these mechanisms are not without drawbacks. Pivoting the charge electrode plate requires a substantial clearance in the printer structure. The translational mechanism, on the other hand, is one in which the charge electrode plate is mounted on a spring arm and cammed out of its operating position. It will be appreciated that a spring mounting mechanism may be subject to undesirable vibration and, additionally, the position of the charge electrode plate may be subject to dimensional inaccuracies due to temperature variations.
IBM Technical Disclosure Bulletin, Vol. 19, No. 8, January 1977, pages 3216 and 3217, discloses an ink jet printer in which a pair of charge electrode plates are moved laterally into and out of operating positions after start-up and prior to shutdown, respectively. Additionally, a pair of catchers, positioned outwardly of the two parallel rows of jet drop streams during operation of the printer, are moved laterally together into contact at start-up and shutdown to prevent splattering of the ink on the print receiving medium.
Keur, U.S. Pat. No. 4,160,982 discloses an ink jet printing system having a catcher which is positioned in line with the non-deflected jet drop stream during printing and which is raised to abut directly the print head during start-up and shutdown. The charging and deflection electrodes are pivotally mounted such that they may be moved out of the way to permit this movement of the catcher.
In Paranjpe et al U.S. Pat. No. 4,238,805, an ink jet printing system is shown which includes a pair of catchers which are pivotally mounted to be movable into positions in which substantially all of the drops from a pair of rows of jet drop streams strike the catchers during start-up and shutdown. The mechanical linkage arrangement which pivots the catchers and, additionally, which translates charge electrode plates into and out of operating positions is, however, relatively complicated. It will be appreciated that it is desirable to limit movement of printer elements as much as possible in an ink jet printing system so as to enhance dependability of the system.
Schwob U.S. Pat. No. 4,286,272 shows the start-up arrangement in which the drops from the jet drop streams are initially deflected to a catcher structure so as to prevent printing at the time of start-up. The catcher structure is not moved between start-up and the ordinary printing operation. Deflection of the jet drop streams results from lateral fluid movement through the print head which imparts a lateral velocity component to the drops in the jet drop streams. This arrangment requires a relatively large fluid manifold inlet to the print head and outlet from the print head such that the lateral fluid flow velocity component can be imparted to all of the jet drop streams along the entire row of streams.
Accordingly, it is seen that there is a need for a simple, reliable, and compact ink jet printer in which start-up and shutdown of the printer are facilitated without the need for movable catchers and charge electrode assemblies.